Abstract
Introduction
Osteolysis caused by wear of the ultrahigh molecular weight polyethylene (UHMWPE) often leads to failure. Cross-linking improves wear, but also produces residual free radicals that decrease oxidative stability. In vitro studies have shown that the anti-oxidative properties of vitamin E UHMWPE stabilize free radicals while retaining the physical and chemical properties of UHMWPE. The porous surface of the Regenerex™ shell was developed for improved bone in-growth fixation. The increased porosity of the Regenerex™ shell promotes early bony in-growth with the goal of greater long-term stability. The purpose of this study was to evaluate vitamin E infused polyethylene (VEPE) wear and stability of acetabular and femoral components using RSA.
Methods
58 patients (64 observed hips), all with osteoarthritis, gave informed consent to participate in a 5 year RSA study. Each patient received a VEPE liner, a Regenerex™ acetabular shell, and an uncemented stem with either a 32mm or 36 mm cobalt chrome femoral head. Tantalum beads were inserted into the VEPE, the pelvic and the femoral bone to measure head penetration into the polyethylene, and shell and stem stability over time, using RSA. RSA radiographs were scheduled immediately postoperatively (up to 6 weeks) and 6 months, 1, 2, 3, and 5 years after surgery. The Wilcoxon signed-ranks nonparametric test was used to determine if changes in penetration or migration were significant over time at p≤0.05.
Results
58 hips have been followed for 6 months, 55 at 1 year, 52 at 2 years, 47 at 3 years and 18 at 5 years. The 36mm cohort data is not reported at this time due to insufficient follow-up. The median± standard error (SE) superior head penetration into the polyethylene was 0.05±0.01mm at 2 years, 0.05±0.01mm at 3 years, and 0.05±0.02 mm at 5 years. The acetabular components had a median± SE cup translation in the proximal direction of 0.09±0.03mm at 2 years, 0.04±0.04mm at 3 years, and 0.06±0.06mm at 5 years. The median± SE cup rotation was −0.09±0.16 degrees at 2 years, −0.02±0.15 degrees at 3 years, and 0.30±0.20 degrees at 5 years. There was a statistically significant difference in cup rotation between the 6 month and 1 year intervals (p=0.007), but no significant differences in translation or head penetration. The median± SE stem distal migration was 0.08±0.07mm at 2 years, 0.05±0.23mm at 3 years, and 0.02±0.17mm at 5 years, with a significant difference between the 6 month and 3 year intervals (p=0.029).
Discussion
The VEPE liners show low head penetration at 5 years. The early head penetration, probably due to creep, is substantially lower relative to that reported for non-vitamin E stabilized UHMWPE measured by similar techniques. At 5 years, all acetabular components were stable, with the early significant difference in rotation at 1 year likely due to early settling of the cup. This study documents the longest-term evaluation of in vivo wear performance of vitamin E stabilized UHMWPE. The stability of the Regenerex™ shell and femoral stem shows promise for long-term survivorship.
